The invention relates to an abrasion-resistant, multilayer, biaxially oriented carrier film for magnetic information media, composed of a base layer A, comprising a thermoplastic polymer matrix made of essentially linear aromatic polyester, containing finely distributed solid particles to improve its surface roughness, which determines the slip properties of the film, and of at least one copolyester cover layer B, applied to one of the two surfaces of base layer A.
Biaxially oriented films of polyesters, especially polyethylene terephthalate, because of their superior properties such as tensile strength, tear resistance, modulus of elasticity, transparency, chemical resistance, thermal resistance and the like are widely used in many many technical areas especially for magnetic recording media.
The polyester films, however, must have a specific combination of properties. Thus, polyester films to be processed into magnetic tapes to be used for audio, video, or computer technology must have a low coefficient of friction and a high abrasion resistance and must be insensitive to deterioration of their electromagnetic transmission properties.
To improve the properties of polyester films, methods are already known for incorporating fine particles of an inert material in the films, for example by using one of the methods listed below:
(1) In general the residue of metallic compounds, used as re-esterification catalysts, is precipitated in the system by special measures in the form of finely distributed particles (called the "precipitation method"); and/or PA0 (2) Microparticles of an inorganic or organic compound with different particle sizes are added from the exterior (called the "addition method"). PA0 (i) a polyester substrate layer having from 0.001 weight percent to 10 weight percent, based upon the weight of the polymer forming said substrate layer, of finely distributed solid particles having an average particle size of from 0.001 to 20 microns; and PA0 (ii) a copolyester cover layer applied to at least one of the two surfaces of said polyester substrate, said copolyester cover layer having a maximum thickness of 40% of the thickness of said polyester substrate, and having from 0 to 10 weight percent, based upon the weight of the polymer forming said copolyester cover layer, of finely distributed solid particles having an average particle diameter of less than 0.3 micron. PA0 (i) separately plasticizing the polyester and copolyester resins which are to form the layers of said film; PA0 (ii) combining said plasticized resins and extruding said resins in sheet-like form through a coextrusion nozzle; PA0 (iii) contacting the combined, multilayer extrudate upon a cooling roller, thereby cooling said extrudate in an unstretched state; PA0 (iv) biaxially orienting said cooled multilayer extrudate to form a multilayer polyester film; PA0 (v) heat-fixing said biaxially oriented polyester film, PA0 (A) regulating the speed at which the plasticized polyester resins are permitted to pass to said co-extrusion nozzle, or PA0 (B) regulating the speeds of the extruders. PA0 1. The surface roughness remaining after layer B is applied, which is caused by the particle-produced surface structuring of layer A, said structuring projecting partially through cover layer B or which in any case causing a certain surface structuring of cover layer B as well, must be sufficiently great that frictional characteristics of the film result therefrom which have a positive influence both on the winding of the film and on the running behavior of magnetic tapes made from the film. PA0 2. The particles must be sufficiently far removed from the free surface of cover layer B to prevent the particles from being torn loose and thus prevent abrasion.
Single-layer films, containing organic particles in a fine distribution, are described, for example, in German Offenlegungschrift No. 30 19 073. Films containing inorganic particles to improve their slip properties are described in German Auslegeschrift No. 21 33 895 and German Pat. No.26 47 713. These films are single-layer films and therefore have a surface roughness on both surfaces which depends upon the quantity of particles added in each case as well as the particle diameter.
To reduce the surface roughness on the surface of the film intended to receive a magnetic coating, multilayer films are also manufactured consisting of a first layer of thermoplastic polymer without particles added, which has a comparatively smooth surface, and a second thinner layer, provided in the usual fashion with particles.
It has been found that the greatest disadvantage of particles incorporated into conventional polyester films for the purpose of improving the slip properties of the film lies in their lack of affinity for the polymer material and their tendency to form agglomerates. For this reason, the friction between such conventional films with one another or between such a film and another material causes the particles to be removed from the film or films, leading for example to the formation of white dust or deposits on the films which are used for magnetic tapes. These deposits are highly undesirable since they are deposited in the equipment for playing the magnetic tapes, causing damage thereto.
If the particles which are normally distributed uniformly in the polymers forming the individual layers are deposited very close to the surface of the layer, friction during winding or rewinding of tapes already provided on one side with a magnetic coating or of films which have not yet been coated readily leads to separation of the polymer layer flakes covering the particles, which are extremely thin, and to release of the embedded particles. The extremely thin polymer layer flakes however adhere very tightly by electrostatic charge either to the film or to the applied magnetic layer, leading to extremely undesirable disturbing effects, to adhesion losses if the magnetic coating had not yet been applied before the flakes came loose, or to signal losses if the magnetic coating had already been applied before the flakes came loose. In addition, the released solid particles penetrate the magnetic coating causing deformations and eventually, signal losses or distortions as well.
It has also been found in practice that scratches form under mechanical stress in the surfaces of films including those films not provided with embedded particles, and that small polymer particles are released from the film in the vicinity of these scratches and can also cause the disadvantages already mentioned hereinabove.
These disturbing effects, generally covered by the term "abrasion behavior" can, in the least favorable case, render magnetic tapes manufactured from the carrier materials listed useless for their designated use as video, audio, or computer tapes. The same applies to flexible disks or floppy disks.
Therefore the object of the present invention is to proceed on the basis of the demonstrated state of the art to provide a carrier film for magnetic information media, which (1) has a low surface roughness on the surface designated to receive the magnetic coating, (2) has a relatively high surface roughness which determines the slip properties on the surface opposite the magnetic coating, (3) has sufficiently high strength lengthwise and crosswise to allow manufacture of extremely thin tapes with extremely high signal density, and (4) exhibits an excellent abrasion behavior, so that even when subjected to extreme frictional stress, no deposits can be seen on the film or magnetic coating surface.